The present invention relates to a technique used for a digital communication network and, more particularly, a technique of setting an alternate route in the event of a fault.
A conventional technique will be described below with reference to FIGS. 6 to 9. FIG. 6 shows a network arrangement constituted by only digital crossconnect systems. FIG. 7 shows the arrangement of a conventional digital crossconnect system. FIG. 8 shows a network arrangement constituted by digital crossconnect systems (DCSs), line terminating multiplexers (LTMs), and add/drop multiplexers (ADMs). FIG. 9 shows a case wherein a different path is present on an alternate route. According to the conventional automatic communication network restoration scheme disclosed in Japanese Patent Laid-Open Nos. 60-22848 and 64-23647, as a communication network as a target for the automatic communication network restoration scheme (to be simply referred to as an automatic restoration scheme hereinafter) for a fault end remedy and a fault circuit end remedy, a network like the one shown in FIG. 6, is used which is constituted by only digital crossconnect systems (to be referred to as DCSs hereinafter), is assumed. Each DCS is constituted by a pair of line terminating units for terminating an optical sync signal frame, obtained by multiplexing a plurality of STS1 signals (STS: Synchronous Transmission System), and demultiplexing the frame into STS1 signals, a sync signal switch unit for crossconnecting the STS1 signals from the pair of line terminating units, and a control unit for controlling these two types of elements. For this reason, for example, a loss of signal (to be referred to as a LOS hereinafter) as a communication alarm is used as a starting trigger for starting communication network restoration processing of an automatic restoration scheme for a fault remedy.
FIG. 7 shows the internal arrangement of each DCS. Each of the pair of line terminating unit of a DCS 101 is constituted by a line terminating unit 104, a line protection switching unit 105, an STS pointer processing unit 106, and an STS path terminating/monitoring unit 107. The sync signal switch unit is constituted by an STS path switch 113. The control unit is constituted by a crossconnect agent unit 108, a TRANS process unit 109 for performing automatic restoration scheme (to be referred to as a TRANS hereinafter) processing, a management information routing managing unit 110, a pair of data communication channel terminating units 111, and a management information managing unit 112 for managing management information such as line termination point management information, path termination point management information, and a sender/chooser list 114 set by a network management system (NMS). Upon reception of an acknowledgement of a P-AIS (Path Alarm Indication Signal) from the STS pointer processing unit 106, the crossconnect agent unit 108 acquires sender/chooser information held in the management information managing unit 112. If an alarm termination point is present at a sender, the crossconnect agent unit 108 starts the TRANS process unit 109 to automatically search for and setting an alternate route.
Assume that in this automatic restoration scheme, a fault has occurred at a portion indicated by a mark "X" on a current route indicated by a dotted line in FIG. 6. In this case, if the alternate route, indicated by the chain lines, between the DCSs 101 at the two ends of the communication network including the fault portion, i.e., a communication network as a target for communication network restoration, is constituted by only the DCSs 101, no problem is posed in the use of a LOS, which is used when a communication network is to be restored by a fault end remedy, as a starting trigger.
An actual communication network, however, includes line terminating multiplexers (to be referred to as LTMs hereinafter) 131, each for terminating an optical sync signal frame obtained by multiplexing a plurality of STS1 signals, and add/drop multiplexers (to be referred to as ADMs hereinafter) 121, each capable of terminating an optical sync signal frame, obtained by multiplexing a plurality of STS1 signals, and dropping some of the STS1 signals, as shown in FIG. 8, as well as the DCSs 101 constituting the network. In the communication network shown in FIG. 8, which includes the LTMs 131 and the ADMs 121, a LOS generated in the event of a fault (optical fiber disconnection) in the communication network may be absorbed by the LTM 131 or the ADM 121 in the subsequent stage. As a result, this communication alarm may not be transferred to the DCS 101 incorporating the automatic restoration scheme. If, therefore, a LOS is used as a starting trigger for an automatic restoration scheme, automatic restoration of a communication network may not be executed.
In addition, since a communication network as a target for a fault end remedy and a circuit end remedy includes not only the DCSs 101 but also the ADMs 121, as described above, a different path like the one indicated by the chain double-dashed line in FIG. 9 may be set on the alternate route, indicated by the chain line, in the automatic restoration scheme. In this case, a transmission route included in the different path cannot be used as an alternate route. According to the internal arrangement of the existing DCS 101, it cannot be detected whether this different path is newly set. For this reason, when a fault such as an optical fiber disconnection occurs, the DCS 101 may perform alternate route setting processing while considering that this route is not used.